Led lamp failure detection circuit and method

ABSTRACT

An LED fault detection circuit is provided for detecting a fault in at least one LED. The circuit may include a test voltage generator for applying at least one test voltage level to the at least one LED, wherein the at least one test voltage level is less than a nominal operating voltage range; a current measuring circuit for measuring at least one current draw by the at least one LED at the at least one test voltage level; and a processing circuit coupled to the test voltage generator for causing the at least one test voltage level to be applied to the at least one LED, the processing circuit coupled to the current measuring circuit for detecting a fault in the at least one LED when the at least one measured current draw has a profile that does not correspond to an expected current draw profile.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority under 35 U.S.C. § 119(e) upon U.S. Provisional Patent Application No. 62/580,787, entitled “LED LAMP FAILURE DETECTION SYSTEM AND METHOD” filed on Nov. 2, 2017, by Thomas J. Veenstra et al., the entire disclosure of which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention generally relates to an electronic circuit for detecting a failure in an LED lamp, and more particularly, to an LED lamp failure detection circuit for vehicle applications.

BACKGROUND

Numerous lamp applications require some sort of automated response if failure mode conditions exist when they are in use. Some failure mode conditions, however, are not detectable under normal operating conditions without the addition of highly integrated monitoring sub circuits.

LED (light emitting diode) lamps are commonly configured such that multiple LEDs are wired in series with current regulator (active or passive). Within this lamp topology, it is possible for one or more LEDs to become short circuited. In this situation the lamp will continue to draw the same nominal current at the same nominal operating voltage in comparison to a normal functioning lamp. The current state of the art method for detecting these failure modes is to embed a monitoring circuit which measures the voltage across the string of series LEDs. It is common that these lamps have multiple strings of LEDs, in which case, the monitoring circuit either needs to monitor each string individually, or have multiple monitoring circuits which communicate with one another. These circuits can quickly become complicated and costly. Additionally, these circuits can be highly sensitive to EMC due to their long monitoring traces and the lockout response condition can be a serious error when false detections occur under EMC conditions.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, an LED fault detection circuit is provided for detecting a fault in at least one LED, the circuit comprising: a test voltage generator for applying at least one test voltage level to the at least one LED, wherein the at least one test voltage level is less than a nominal operating voltage range; a current measuring circuit for measuring at least one current draw by the at least one LED at the at least one test voltage level; and a processing circuit coupled to the test voltage generator for causing the at least one test voltage level to be applied to the at least one LED, the processing circuit coupled to the current measuring circuit for detecting a fault in the at least one LED when the at least one measured current draw has a profile that does not correspond to an expected current draw profile.

According to another embodiment of the present invention, a method of detecting a fault in at least one LED is provided comprising: applying at least one test voltage level to the at least one LED, wherein the at least one test voltage level is less than a nominal operating voltage range; measuring a current draw by the at least one LED at each of the at least one test voltage level; and detecting a fault in the at least one LED when the measured current draw has a profile that does not correspond to an expected current draw profile.

According to another embodiment of the present invention, an LED fault detection circuit is provided for detecting a fault in an LED lamp having at least one string of LEDs, the circuit comprising: a test voltage generator for applying a plurality of different test voltage levels to the at least one string of LEDs, wherein the plurality of different test voltage levels are each less than a nominal operating voltage range; a current measuring circuit for measuring a current draw by the at least one string of LEDs at each of the plurality of different test voltage levels; and a processing circuit coupled to the test voltage generator for causing the plurality of different test voltage levels to be applied to the at least one string of LEDs, the processing circuit coupled to the current measuring circuit for detecting a fault in the at least one string of LEDs when the measured current draws have a profile that does not correspond to an expected current draw profile.

These and other features, advantages, and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims, and appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:

FIG. 1 is an electrical circuit diagram in block form showing the LED lamp fault detection circuit;

FIG. 2 is a plot of lamp current draw versus drive voltage for three LED lamps;

FIG. 3 is an electrical circuit diagram in schematic form showing examples of a test voltage generator, a processing circuit, and a main power switch of the LED lamp fault detection circuit shown in FIG. 1;

FIG. 4 is an electrical circuit diagram in schematic form showing an example of a current measuring circuit of the LED lamp fault detection circuit shown in FIG. 1;

FIG. 5 is a flowchart showing an example of an algorithm that may be executed by the processing circuit of the LED lamp fault detection circuit shown in FIG. 1; and

FIG. 6 is an electrical circuit diagram in schematic form showing alternative examples of a test voltage generator, a processing circuit, and a current measuring circuit of the LED lamp fault detection circuit shown in FIG. 1.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present illustrated embodiments reside primarily in combinations of method steps and device components related to an electronic circuit for detecting faults in an LED lamp. Accordingly, the circuit components and method steps have been represented, where appropriate, by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present disclosure so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. Further, like numerals in the description and drawings represent like elements.

It is to be understood that the specific devices and processes illustrated in the attached drawings and described in the following specification are simply exemplary embodiments of the inventive concepts defined in the appended claims. Hence, specific dimensions and other physical characteristics relating to the embodiments disclosed herein are not to be considered as limiting, unless the claims expressly state otherwise.

For purposes of this disclosure, the term “coupled” (in all of its forms, couple, coupling, coupled, etc.) generally means the joining of two components (electrical or mechanical) directly or indirectly to one another. Such joining may be stationary in nature or movable in nature. Such joining may be achieved with the two components (electrical or mechanical) and any additional intermediate members being integrally formed as a single unitary body with one another or with the two components. Such joining may be permanent in nature or may be removable or releasable in nature unless otherwise stated.

FIG. 1 shows an example of an LED lamp fault detection circuit 10, which may comprise a test voltage generator 40 for applying at least one test voltage level to at least one LED 20, a current measuring circuit 60 for measuring at least one current draw by the at least one LED 20 at the at least one test voltage level, and a processing circuit 80 coupled to the test voltage generator 40 for causing the at least one test voltage level to be applied to the at least one LED 20. The processing circuit 80 is also coupled to the current measuring circuit 60 for detecting a fault in the at least one LED 20 when the at least one measured current draw has a profile that does not correspond to an expected current draw profile.

The processing circuit 80 may also be coupled to a main power switch 90 for causing a nominal operating voltage level falling within a nominal operating voltage range to be applied to the at least one LED 20. The at least one test voltage level generated by the test voltage generator 40 is less than the nominal operating voltage range. A power source 30 may be provided to supply power to the various components of circuit 10 and may vary considerably in construction depending on the environment in which the circuit 10 is applied. The power source may be an external source, such as a vehicle battery, for example.

The LED lamp fault detection circuit 10 may be used to detect a fault in a lamp having a single LED, a string of LEDs, or a plurality of strings of LEDs. In addition, the LED lamp fault detection circuit 10 may be coupled to multiple lamps so as to simultaneously detect a fault in a plurality of lamps. Thus, as used herein, “at least one LED” shall encompass a single LED, a plurality of LEDs, a string of LEDs, or a plurality of strings of LEDs whether in a single lamp or plurality of lamps.

In the proposed circuit 10 and method, a voltage vs. current profile is characterized, analyzed, and used to identify failure modes. As noted above, the proposed circuit and method function by applying at least one test voltage level to the LED lamp and measuring the current draw of the LED lamp. A shorted LED fault condition may then be detected based on abnormal current draw conditions at any of the at least one test voltage levels.

FIG. 2 shows the current draws of three examples of LED lamps having a plurality of LEDs over various voltage levels. The first LED lamp is a “good” circuit having no shorted LEDs, the second LED lamp is a “failed circuit” having one shorted LED, and the third LED lamp is a “failed circuit” having two shorted LEDs. In a nominal operating voltage range (e.g., 11V-13V, for a vehicle), the current draws for both good and failed circuits are identical. Thus, if one merely tested the current draw of an LED lamp at the nominal operating voltage level, they could not distinguish between good and failed circuits. However, the inventors have discovered that the failed circuits follow different ramp-up current draw profiles between 2 and 9 volts, for example, than the good circuit. Accordingly, by selecting at least one test voltage level that is less than the nominal operating voltage level (and particularly less than 9V or even 7V in the example shown), one could distinguish a failed circuit from a good circuit based on the measured current draw. Thus, to detect an abnormal current draw profile, one could apply a single test voltage of 7V, for example, and compare the measured current draw to an expected current draw of a good circuit and determine if the measured current draw is within a threshold range of the expected current draw. If it is not, a fault would be identified in the at least one LED.

Another method to detect an abnormal current draw profile would be to apply a plurality of different test voltage levels while measuring the current draw at each of the different test voltage levels. One could then compare the measured current draw for each test voltage level with the expected current draw at that test voltage level for a good circuit. If any one current draw differs from the expected current draw at any one of the test voltage levels, a fault may be identified. Alternatively, a more robust solution would be to either average the differences between the measured and expected current draws and/or compare the slopes of the voltage vs. current profiles for measured current draws and expected current draws. In a broad sense, a fault may be detected in the at least one LED 20 when the at least one measured current draw has a profile that does not “correspond to” an expected current draw profile. As used herein, a measured profile “corresponds to” an expected profile if the measured profile exhibits properties within predefined limits of the expected profile. Those properties may be any one of slope, rate of change of slope, and/or distance/difference/offset at any one voltage or current level or over a plurality of voltage or current levels, or even a turn on voltage at which the LED lamp first begins to draw current. The predefined limits may be determined based on the particular application. For example, the expected current draw profile for a good circuit may vary based upon the number of LEDs, the color of LEDs, configuration of LED strings, other components coupled to the LEDs, and/or environmental conditions. Thus, one would first construct a good circuit and measure the current draw over the different test voltage levels to be used, and then would form the expected current draw profile from the measured current draws. Optionally, one could then short out one LED and measure the current draw over the different test voltage levels to be used. By comparing the current draw profile of the failed circuit to the expected current draw profile, one could then select a characteristic of the profiles that best distinguishes them from one another as well as predefined limits for those characteristics. For example, in FIG. 2, one could select the turn on voltage as the characteristic used to distinguish a failed circuit from a good circuit. Here a failed circuit would begin to draw current at either about 3V or 5V whereas a good circuit does not begin to draw current until about 6.5V. In this case the predefined limit may be selected to be 6V. However, the predefined limit may be selected based on other factors such as the extent to which the current draw profile of a good circuit varies with environmental conditions such as temperature. Thus, if a current draw profile of a good circuit varies such that the turn on voltage falls to 5.8V at a first temperature and the current draw profile of a failed circuit varies such that the turn on voltage rises to 5.5V at a second temperature, one may select the predefined limit for turn on voltage to be 5.65V to enable discrimination regardless of temperature. This example is provided to demonstrate that there are no hard and fast rules for selecting the characteristic or the predefined limits for determining if a measured current draw profile corresponds to an expected current draw profile.

FIG. 3 shows examples of the test voltage generator 40, the processing circuit 80, and the main switch 90. As shown, an optional power conditioner circuit 95 is provided to perform certain conditioning of the power supplied from the power source 30. The particular conditioning to be performed depends upon the application. For example, when applied in a vehicle, the power source 30 may be the vehicle battery and/or the vehicle ignition and the power conditioner circuit 95 may include reverse polarity protection and surge protection.

The test voltage generator 40 may include an adjustable voltage regulator U1 that receives a voltage Vpos from either the power source 30 or power conditioner 95. The adjustable voltage regulator U1 may be a model LM317LIDR voltage regulator available from Texas Instruments, for example. The adjustable voltage regulator U1 outputs a voltage that is a fraction of the input voltage Vpos as determined by an adjustment voltage supplied to the ADJ input of the adjustable voltage regulator U1. The adjustment voltage may be set by a voltage divider formed of resisters R1 and R2. Alternatively, the adjustment voltage may be selected by processing circuit 80 via a digital-to-analog converter. In this latter case, the processing circuit 80 may select one or more different test voltages that may then be applied to the at least one LED 20.

The test voltage generator 40 may further include a Vtest switch Q1 that selectively applies the output of the adjustable voltage regulator U1 to the at least one LED 20 depending on the voltage applied to its gate. The gate of switch Q1 is coupled to a switch Q2 that selectively couples the gate of switch Q1 to ground when a test enable signal (TEST-EN) is received from the processing circuit 80. Thus, the processing circuit 80 may control when the test voltage level output from the test voltage generator 40 is to be applied to the at least one LED 20.

The test voltage generator 40 may also include a pair of capacitors C1 and C2 for smoothing the output of the adjustable voltage regulator U1.

Processing circuit 80 may include a processor U2 or any other form of analog and/or digital circuitry that can perform the intended functions. An example of a suitable processor U2 is the PIC16F1503 available from Microchip. Processor U2 may include non-transitory computer-readable medium that is capable of storing software instructions that, when executed by the processor U2, cause the processor U2 to perform certain method steps as described below in connection with FIG. 5. This method will be described following a description of the remainder of FIG. 3 and the entirety of FIG. 4.

The main switch 90 is an optional component that may be used to apply a nominal operating voltage to the at least one LED lamp 20. As shown, the switch 90 may include power switch U3 that receives Vpos as an input voltage and selectively supplies Vout to the at least one LED 20. The power switch U3 may be a smart high-side power switch such as model BTS4175 available from Infineon. Power switch U3 includes a Vin terminal that may be coupled to receive a main enable (MAIN_EN) signal from the processing circuit 80. Thus, the processing circuit 80 may enable or disable the power switch 90 from supplying the nominal operating voltage to the at least one LED 20. Accordingly, when enabling the test voltage generator 40 to supply the at least one test voltage level to the at least one LED 20, the processing circuit 80 may disable the main power switch 90 so that it does not supply the nominal operating voltage. Likewise, when enabling the main power switch 90 to supply the nominal operating voltage to the at least one LED 20, the processing circuit 80 may disable the test voltage generator 40 so that it does not supply the at least one test voltage level.

As described with respect to FIG. 4 below, the current measuring circuit 60 is disposed between the at least one LED 20 and the power line from the main power switch 90 and the test voltage generator 40 so as to measure the current drawn by the at least one LED 20. The processing circuit 80 may record the measured current draw at each of a plurality of test voltage levels.

An example of a current measuring circuit 60 is shown in FIG. 4. The current measuring circuit 60 may include a resistor R4 that is coupled between the at least one LED 20 and the main power switch 90 and the test voltage generator 40. The current measuring circuit 60 measures the voltage across the resistor R4 so as to determine the current flowing therethrough using the property I=V/R where R is the known resistance of R4. The current measuring circuit 60 may include a current/power monitor U4, which may, for example, be a bidirectional current/power monitor, such as model number INA219A available from Texas Instruments. The current/power monitor U4 includes two terminals IN+ and IN− that are coupled to opposite sides of resistor R4 through resistors R5 and R6, respectively. A capacitor C3 may be coupled between resistor R5 and ground. A capacitor C4 may be coupled between resistor R6 and ground. A capacitor C5 may be coupled between terminals IN+ and IN−. A supply voltage of +5V, for example, may be applied to the Vs terminal of current/power monitor U4. A capacitor C6 may be coupled between the Vs terminal and ground. The supply voltage may also be applied to resistors R7 and R8 that are coupled to output terminals SDA and SCL, respectively, of current/power monitor U4. The output terminals SDA and SCL are coupled to respective input terminals SDA and SCL of processor U2 (FIG. 3) via resistors R9 and R10, respectively. It is through output terminal SDA that the measured current draw is delivered to processing circuit 80.

Having described the LED lamp fault detection circuit 10 and examples of its components, an algorithm in the form of method 100 is described below with respect to FIG. 5.

The particulars of the method 100 may vary depending upon the application. For example, three different applications may be detecting a fault in an LED brake light, an LED turn signal, or an LED lamp for an illuminated vehicle logo.

The exemplary method 100 may begin with the processing circuit 80 determining if the LED lamp is to be activated (step 102). This may occur by the processing circuit 80 receiving an external signal via a dedicated line or a vehicle bus that the brake pedal has been activated, the turn signal has been activated, or the logo lamp is to be turned on. Alternatively, the processing circuit 80 may sense the input voltage Vin from the power source 30 in applications where the power source 30 supplies power whenever the lamp is to be activated. If no such activation signal is received, step 102 is repeated until such time that an activation signal is received.

When an activation signal is received, the processing circuit 80 may then determine whether to initiate a test of the LED lamp in step 104. This may occur each time the LED lamp is to be activated or just the first time the LED lamp is activated during an ignition cycle again depending upon the application. If no test is to be performed, the processing circuit 80 advances to step 116 in which it disables the test voltage generator 40 via a TEST-EN signal and enables the main power switch 90 via a MAIN-EN signal so that the nominal operating voltage level is applied to the at least one LED 20 of the LED lamp.

If the processing circuit 80 determines in step 104 that a test is to be initiated, it advances to step 106 in which it disables the main power switch 90 via a MAIN-EN signal and enables the test voltage generator 40 via a TEST-EN signal so that a test voltage level is applied to the at least one LED 20 of the LED lamp. Next the processing circuit 80 reads the measured current draw from the current measuring circuit 60 in step 108. In step 110, the processing circuit 80 determines if all of the test voltage levels have been applied to the at least one LED lamp 20. If not all test voltage levels have been applied, the processing circuit 80 causes test voltage generator 40 to apply a different test voltage level to the at least one LED 20 in step 112 and returns to step 108 to read the measured current draw from the current measuring circuit 60. Once all the test voltage levels have been applied, the processing circuit 80 advances to step 114 in which it determines whether the measured current draw profile at the test voltage level(s) corresponds to the expected current draw profile of a good circuit. If the profiles adequately correspond to one another, the processing circuit determines that there is no fault in the LED lamp and proceeds to step 116 where it disables the test voltage generator 40 via a TEST-EN signal and enables the main power switch 90 via a MAIN-EN signal so that the nominal operating voltage level is applied to the at least one LED 20 of the LED lamp. This state is maintained until the LED lamp is deactivated and the main power switch is disabled so no voltage is applied to the LED lamp.

If, in step 114, the processing circuit 80 determines that the profiles do not adequately correspond to one another, the processing circuit 80 determines that a fault is present in the LED lamp in step 118. The processing circuit 80 may be programmed to take any one or more of different actions when a fault is detected. The action(s) taken may depend on what the vehicle manufacturer wants to happen. For example, if a fault is detected, the processing circuit 80 may be configured to disable any future activation of the LED lamp. In this way, the driver may realize the lamp is in need of replacement. On the other hand, it may be necessary to keep the LED lamp functioning as best as possible. For example, for an LED brake lamp, it may be desirable to maintain some level of operation of the brake lamp for safety purposes. In this case, a warning signal may be generated for the driver that the brake lamp should be replaced. Actions taken for a faulting turn signal lamp may be similar to those for a brake lamp.

Although a main power switch 90 is shown and described above, it is possible to incorporate its function into the test voltage generator 40 so that the processing circuit 80 may simply instruct the test voltage generator 40 to output a nominal operating voltage level. FIG. 6 shows one example where the main power switch 90 is not be used.

FIG. 6 shows an alternative embodiment, wherein the fault detection circuit 10 includes a test voltage generator 40, a current measuring circuit 60, and a processing circuit 80. In this case, the current measuring circuit 60 includes just the resistor R4. the processing circuit 80 includes a pair of inputs for sensing the voltage across the resistor R4 and calculates the measured current from the measured voltage. The test voltage generator 40 includes a switch Q3 that selectively connects a charging capacitor C6 to the input power through a resistor R11. The switch is activated by an enable signal sent from the processing circuit 80. In this example, the processing circuit 80 is coupled to the Vin input so as to sense when the power source 30 is supplying power thereby indicating that the lamp is to be activated. When Vin is sensed, the processing circuit 80 enables the switch Q3 to begin charging the capacitor C6. As the capacitor C6 charges, the charge voltage is applied to the at least one LED 20 and the processing circuit 80 takes several current measurements of the current draw by the at least one LED 20 at various test voltages. When the capacitor C6 is fully charged, the charge voltage reaches the nominal operating voltage level. The processing circuit 80 can then compare the measured current draw profile to the expected profile and identify a fault in the manner discussed above.

The proposed fault detection circuit and method as described above can detect the aforementioned failed LED lamp conditions without the need for highly integrated monitoring sub circuits. The circuit and method can be used to monitor multiple strings of LEDs simultaneously without the need for long sensitive monitoring traces. Furthermore, the proposed method allows the monitoring circuit and algorithm to be either embedded in the lamp or external to the lamp. Whereas, the existing state of the art requires the monitoring sub circuits to be embedded in the lamp.

The LED lamp fault detection circuit 10 reduces the total circuit complexity, cost, and false failure sensitivity for monitoring these conditions. Additionally, the LED lamp fault detection circuit 10 can be set up as an external module, which allows it to be used as an add-on feature. In some applications, this would eliminate the need to have unique lamp sets.

It is important to note that the construction and arrangement of the elements of the embodiments as shown in the exemplary embodiments is illustrative only. Although only a few embodiments of the present innovations have been described in detail in this disclosure, those skilled in the art who review this disclosure will readily appreciate that many modifications are possible (e.g., variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters, mounting arrangements, use of materials, colors, orientations, etc.) without materially departing from the novel teachings and advantages of the subject matter recited. For example, elements shown as integrally formed may be constructed of multiple parts or elements shown as multiple parts may be integrally formed, the operation of the interfaces may be reversed or otherwise varied, the length or width of the structures and/or members may be varied, the nature or number of adjustment positions provided between the elements may be varied. Accordingly, all such modifications are intended to be included within the scope of the present innovations. Other substitutions, modifications, changes, and omissions may be made in the design, operating conditions, and arrangement of the desired and other exemplary embodiments without departing from the spirit of the present innovations.

It will be understood that any described processes or steps within described processes may be combined with other disclosed processes or steps to form structures within the scope of the present invention. The exemplary structures and processes disclosed herein are for illustrative purposes and are not to be construed as limiting.

The above description is considered that of the preferred embodiments only. Modifications of the invention will occur to those skilled in the art and to those who make or use the invention. Therefore, it is understood that the embodiments shown in the drawings and described above are merely for illustrative purposes and not intended to limit the scope of the invention, which is defined by the claims as interpreted according to the principles of patent law, including the doctrine of equivalents. 

What is claimed is:
 1. An LED fault detection circuit for detecting a fault in at least one LED, the circuit comprising: a test voltage generator for applying at least one test voltage level to the at least one LED, wherein the at least one test voltage level is less than a nominal operating voltage range; a current measuring circuit for measuring at least one current draw by the at least one LED at the at least one test voltage level; and a processing circuit coupled to the test voltage generator for causing the at least one test voltage level to be applied to the at least one LED, the processing circuit coupled to the current measuring circuit for detecting a fault in the at least one LED when the at least one measured current draw has a profile that does not correspond to an expected current draw profile.
 2. The LED fault detection circuit of claim 1, wherein the processing circuit detects that the at least one measured current draw has a profile that does not correspond to the expected current draw profile when the measured current draw for one of the at least one test voltage levels differs from an expected current draw for one of the at least one test voltage levels by an amount that exceeds a threshold.
 3. The LED fault detection circuit of claim 1, wherein the test voltage generator applies a plurality of different test voltage levels to the at least one LED, wherein the plurality of different test voltage levels are less than the nominal operating voltage range, and wherein the current measuring circuit measures a current draw by the at least one LED at each of the plurality of different test voltage levels.
 4. The LED fault detection circuit of claim 3, wherein the processing circuit detects that the at least one measured current draw has a profile that does not correspond to the expected current draw profile when the measured current draw for any one of the plurality of different test voltage levels differs from an expected current draw for the one of the plurality of different test voltage levels by an amount that exceeds a threshold.
 5. The LED fault detection circuit of claim 3, wherein the processing circuit detects that the at least one measured current draw has a profile that does not correspond to the expected current draw profile when the slope of the measured current draw over a test voltage range corresponding to the plurality of different test voltage levels differs from a slope of an expected current draw for the test voltage range by an amount that exceeds a threshold.
 6. The LED fault detection circuit of claim 3, wherein the processing circuit detects that the at least one measured current draw has a profile that does not correspond to the expected current draw profile when a turn on voltage at which a current draw is first measured differs from an expected turn on voltage by an amount that exceeds a threshold.
 7. The LED fault detection circuit of claim 1, wherein the at least one LED comprises a plurality of LEDs.
 8. The LED fault detection circuit of claim 1, wherein the at least one LED comprises a plurality of strings of LEDs.
 9. A method of detecting a fault in at least one LED, the method comprising: applying at least one test voltage level to the at least one LED, wherein the at least one test voltage level is less than a nominal operating voltage range; measuring a current draw by the at least one LED at each of the at least one test voltage level; and detecting a fault in the at least one LED when the measured current draw has a profile that does not correspond to an expected current draw profile.
 10. The method of claim 9, wherein the measured current draw has a profile that does not correspond to the expected current draw profile when the measured current draw for one of the at least one test voltage levels differs from an expected current draw for the one of the at least one test voltage levels by an amount that exceeds a threshold.
 11. The method of claim 9, wherein a plurality of different test voltage levels are applied to the at least one LED, wherein the plurality of different test voltage levels are less than the nominal operating voltage range, and wherein a current draw is measured by the at least one LED at each of the plurality of different test voltage levels.
 12. The method of claim 11, wherein the measured current draw has a profile that does not correspond to the expected current draw profile when the measured current draw for any one of the plurality of different test voltage levels differs from an expected current draw for the one of the plurality of different test voltage levels by an amount that exceeds a threshold.
 13. The method of claim 11, wherein the measured current draw has a profile that does not correspond to the expected current draw profile when the slope of the measured current draw over a test voltage range corresponding to the plurality of different test voltage levels differs from a slope of an expected current draw for the test voltage range by an amount that exceeds a threshold.
 14. The method of claim 11, wherein the measured current draw has a profile that does not correspond to the expected current draw profile when a turn on voltage at which a current draw is first measured differs from an expected turn on voltage by an amount that exceeds a threshold.
 15. The method of claim 9, wherein the at least one LED comprises a plurality of LEDs.
 16. The method of claim 9, wherein the at least one LED comprises a plurality of strings of LEDs.
 17. An LED fault detection circuit for detecting a fault in an LED lamp having at least one string of LEDs, the circuit comprising: a test voltage generator for applying a plurality of different test voltage levels to the at least one string of LEDs, wherein the plurality of different test voltage levels are each less than a nominal operating voltage range; a current measuring circuit for measuring a current draw by the at least one string of LEDs at each of the plurality of different test voltage levels; and a processing circuit coupled to the test voltage generator for causing the plurality of different test voltage levels to be applied to the at least one string of LEDs, the processing circuit coupled to the current measuring circuit for detecting a fault in the at least one string of LEDs when the measured current draws have a profile that does not correspond to an expected current draw profile.
 18. The LED fault detection circuit of claim 17, wherein the processing circuit detects that the measured current draws have a profile that does not correspond to the expected current draw profile when the measured current draw for any one of the plurality of different test voltage levels differs from an expected current draw for the one of the plurality of different test voltage levels by an amount that exceeds a threshold.
 19. The LED fault detection circuit of claim 17, wherein the processing circuit detects that the measured current draws have a profile that does not correspond to the expected current draw profile when the slope of the measured current draw over a test voltage range corresponding to the plurality of different test voltage levels differs from a slope of an expected current draw for the test voltage range by an amount that exceeds a threshold.
 20. The LED fault detection circuit of claim 17, wherein the processing circuit detects that the measured current draws have a profile that does not correspond to the expected current draw profile when a turn on voltage at which a current draw is first measured differs from an expected turn on voltage by an amount that exceeds a threshold.
 21. The LED fault detection circuit of claim 17, wherein the test voltage generator comprises a charging capacitor that is charged when enabled by the processing circuit and that applies a charging voltage as the plurality of different test voltage levels as the charging capacitor charges. 